Intelligent, mentally acute.
"Eulogizing the tight-wittedness of the emperor and the holy congress and praising the good counsel of the emperor...".[1]
[It is said] that there are three parts to learning well: tight-wittedness, memory, acuity. And memory is the preservation of what one has learned; acuity is the swiftness of thought; tight-wittedness is the capacity to use what one has learned to pursue what one has not learned.[2]